A flat disk or "wafer" of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically formed by growing an elongated cylinder or ingot of single crystal silicon and then slicing individual wafers from the cylinder. Multiple layers of conductive material and dielectric material are thereafter built up on the wafer in order to form a multilevel integrated circuit.
The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. The removal of projections and other imperfections is referred to in the art as planarization. Material layers applied to the wafer as integrated circuitry is built must also be planarized in order to produce extremely flat surfaces free of irregularities or projections. To this end, chemical mechanical polishing ("CMP") machines have been developed, and are well known in the art, to provide controlled planarization of semiconductor wafers and layers deposited thereon.
CMP machines generally include one or more wafer carriers or "chucks" which retain and carry wafers to be planarized and which press the front faces of the wafers against the surface of a rotating polishing pad. The wafer carrier is also typically rotated to effect relative lateral motion between the polishing pad and wear and planarization of the wafer face due to frictional contact against the pad. An abrasive slurry, such as a colloidal silica slurry, is usually introduced at the pad-wafer interface in order to augment the planarization process.
A conventional prior art wafer carrier 10 which operates in the manner described above is illustrated in FIG. 1. Carrier 10 includes an upper housing 12, a pressure plate 14 mounted underneath a lower or secondary housing 16, and a bearing assembly 18 disposed between lower housing 16 and upper housing 12. A plurality of fasteners 20 (typically eight) fix pressure plate 14 to lower housing 16. A wafer to be polished is held against a backing pad secured to the planar lower surface of pressure plate 14.
Bearing assembly 18 is a "two-axis" bearing. It permits rocking of lower housing 16 and pressure plate 14 relative to upper housing 12 in both the x- and y-directions in order to maintain the surface of the wafer in parallel contact with the polishing pad even when the pad deviates from planarity. This motion is often referred to as "gimballing", and the "gimbal point" is defined as the intersection of the plane in which the x- and y-axes lie with the vertical central axis of the carrier. The gimbal point of prior art carrier 10, for example, is at point 22. The height of the gimbal point above the lower or backing surface of the pressure plate in conventional prior art carriers is approximately 35 mm. Such a high gimbal point has been found to be detrimental, however, as excessive tipping of the wafer with respect to the polishing pad often occurs, causing uneven edge polishing and detracting from uniform pressure distribution across the wafer.
Conventional carriers are also problematic in that the downward pressure applied by the drive shaft is not ideally distributed across the wafer. In carrier 10, for example, upper housing 12 is connected to outer ring 24 of bearing assembly 18 by fasteners 26; while inner ring 28 of bearing assembly 18 is connected to lower housing 16 by fasteners 30. Hence, the pressure distribution path is as follows: downward pressure applied from the drive shaft is transmitted into upper housing 12, transmitted through fasteners 26 and into outer bearing ring 24, transmitted through bearing assembly 18 to inner bearing ring 28, and transmitted through fasteners 30 to the narrow central body portion 32 of lower housing 16 and pressure plate 14. Consequently, the downward pressure is concentrated at the central portion of the wafer and may effect excessive material removal in the inner diameter portions of the wafer, while bowing and inadequate material removal occurs at the outside diameter portions of the wafer.
In order to promote a more uniform distribution of the pressure load, relatively thick backing plates are typically employed. Increasing the thickness of the backing plate to ensure uniform loading, however, necessarily increases the height of the gimbal point which, in turn, may cause the wafer to tilt with respect to the polishing pad and thereby compromise the polishing process.